A carbon-fiber bundle is formed with multiple carbon-fiber filaments containing carbon as a main component (the component contained in the largest amount). When carbon-fiber-reinforced thermoplastic compositions are formed using such carbon-fiber bundles as reinforcing material for thermoplastic resin, carbon-fiber bundles chopped into sections 3˜15 mm long have been often used. However, in recent years, use of carbon-fiber bundles in the form of continuous fibers is growing in popularity because of high strength utilization rates.
When carbon-fiber bundles in the form of continuous fibers are aligned unidirectionally or woven into a cloth and impregnated with thermoplastic resins to make sheet materials or long-fiber pellets, handling such carbon-fiber bundles is not easy since they tend to become fuzzy or loose during a manufacturing process. When carbon-fiber bundles are used as woven material, whether the carbon-fiber bundles are suitable for being woven into a fabric, whether the woven fabric is easy to handle, and the like are important determinants.
For the reasons described above, to enhance processability of carbon-fiber bundles and physical properties of materials formed with carbon-fiber bundles blended therein, sizing treatment has been conventionally conducted to cause convergence of carbon-fiber bundles. As for sizing treatment, for example, a sizing agent such as polymer material that is compatible with the thermoplastic resin used as the matrix resin is provided on the surface of a carbon-fiber bundle in an approximate amount of 0.2˜5 mass % of the carbon-fiber bundle. Polyolefin resins, polyamide resins, polyester resins and the like are often used as polymer material.
However, since such polymer material is solid at room temperature, when used as a sizing agent by itself, its processability, such as ease of winding or opening fibers required especially for carbon-fiber bundles to be used in the form of continuous fibers, will remain low.
Thus, as an example of a sizing agent that provides excellent processability to carbon-fiber bundles while being compatible with the thermoplastic resin, Patent Literature 1 describes a polymer material mixture of acid-modified polyolefin and a glycol ether-based compound.
Patent Literature 2 describes an example of a sizing agent, formed by mixing acid-modified polyolefin containing an aromatic vinyl compound as its copolymer component and a nonionic surfactant in the form of liquid at 20° C.
Moreover, Patent Literature 3 describes a mixture of polyamide and a nonionic surfactant in the form of liquid at 20° C. as an example where a polymer material other than olefin-based polymers is used as a sizing agent.